1. Field of the Invention
The present invention pertains generally to electrochemical testing assemblies and, more particularly, toward multi-channel rotating disk or ring-disk electrode assemblies and associated testing procedures.
2. Description of Related Art
Generally, a three-electrode electrochemical cell includes a glass vessel holding a working electrode, with a counter electrode and a reference electrode in separate compartments. The electrodes are immersed in a testing solution, such as sulfuric acid, and a reference potential is applied between reference and working electrodes, whereas a current is established between the working and counter electrodes. This setup is used in basic research to investigate the kinetics and mechanisms of the electrode reaction occurring on the working electrode surface.
In some testing applications, a rotating disk electrode (RDE) or rotating ring-disk electrode (RRDE), which are hereinafter collectively referred to as the “RDE/RRDE”, is used as the working electrode in the three-electrode cell. RDE/RRDE are well known in the art and are commercially available from several sources, such as Princeton Applied Research of Oak Ridge, Tenn.
The RDE/RRDE is a specialized hydrodynamic electrode used in the study of the kinetics and mechanisms of electrode reactions for ensuring a known and controllable mass-transfer to the electrode. The mass-transfer is achieved by using a flat disc electrode that is rotated in the testing solution, resulting in a defined hydrodynamic boundary layer. Typically, the RDE/RRDE is coated with a chemical composition whose kinetic parameters in an electrochemical reaction are to be determined. More specifically, thin films of chemical compositions are applied to the electrode surface of the RDE/RRDE, and the inherent electron-transfer characteristics of the chemical compositions are determined in the testing procedure. Conventionally, such tests are very time consuming due to the electrochemical cleaning and gas purging processes, but yield valuable information of the intrinsic electrochemical properties and kinetics so as to warrant further study.
A conventional three-electrode cell testing apparatus 10 is illustrated in FIGS. 1–2, and is shown to include a glass vessel 12 having a generally central port 14 that receives the working electrode 16, a first laterally disposed port 18 that receives a counter electrode 20, and a second laterally disposed port 22 that receives a reference electrode 24. The glass vessel 12 also typically includes a gas inlet port 26 for saturation of the testing solution 30 via a bubbling assembly 27 prior to a test procedure, as well as additional ports 28 for ventilation and gas flowing purposes. The glass vessel 12 holds the testing solution 30, such as sulfuric acid, in which the working electrode 16 is submersed and with which the counter and reference electrodes 18, 22 in separate compartments communicate.
With reference to FIGS. 6A–6B, the working electrode 16 is a RDE (FIG. 6A, 16) or a RRDE (FIG. 6B, 16′), which is conventionally formed as a disk 16a or ring-disk 16a′, 16d′ of electrode material, such as gold, glassy carbon, or platinum, that is imbedded in a rod of insulating material 16b (16b′), such as polytetrafluoroethylene (PTFE) or low expansion oxides. For testing purposes, the electrode material 16a (16a′) is coated (via a plasma deposition process, chemical vapor deposition process, powder ink or the like) with a chemical composition to be tested. The RDE/RRDE shaft 16c, which is electrically connected to the electrode material 16a (16a′, 16d′), extends from the insulating material 16b (16b′) and is mechanically connected to a motor 32 that rotates the working electrode 16 (16′) at a stable, high speed (i.e., 100–8000 rpm), which leads to a well-defined solution flow pattern of mass transfer. In this regard, it is noted that maintenance of the rotary speed is important as this speed is directly related to flow pattern and laminar flow layer parameters at the electrode surface, and thereby affects the electron-transfer properties under investigation. In any event, the RDE/RRDE, which are collectively referred to herein as the working electrode 16, is only used for a single testing procedure.
The reference electrode 24 has a well-known and stable equilibrium electrode potential, and provides a reference point against which the potential of the working electrode 16 is applied. Such reference electrodes are well known in the art and are readily commercially available from several sources, including Princeton Applied Research. Although the reference electrode 24 is received within the glass vessel 12, the reference electrode is typically, and more specifically, disposed within a double bridge tube assembly 25 that is illustrated in FIG. 7.
The double bridge tube assembly, which is hereafter referred to as the reference electrode assembly 25, includes the reference electrode 24 with the first bridge tube 24a and a second bridge tube 24b that protects the testing solution 30 from contamination by the reference electrode solution 30a. Each of the bridge tubes 24a, 24b holds a solution 30a, 30b, respectively, and includes a bridge, which are schematically illustrated and referred to as 24a′, 24b′, respectively. Normally the solution 30b in the second bridge tube 24b is the same as the testing solution 30. The bridges 24a′, 24b′ are typically made from VYCOR frit that prevents contamination of the testing solution 30, which could result from the leakage of the reference solution 30a. Accordingly, the reference solution 30a surrounding the reference electrode 24 is doubly isolated from the testing solution 30 via the bridges 24a′, 24b′. Although the reference electrode 24 is reusable, and may be used for multiple testing procedures, it must be periodically tested to ensure that the electrode potential has not drifted over time.
Strictly speaking, there can be a small change in the potential of the reference electrode 24 depending on the electrolyte because of the presence of a liquid-junction potential. The liquid-junction potential is minimized by the use of high concentration solution, such as potassium chloride, as the solution 30a when the reference electrode 24 is a saturated calomel electrode.
The counter electrode 20 is used to make an electrical connection to the electrolyte or testing solution 30 (sulfuric acid) so that a current can be established between the working electrode 16 and the counter electrode 20. The counter electrode 20 is usually made of inert materials (noble metals or carbon/graphite) to avoid its dissolution. Typically, the counter electrode 20 has high surface area and is disposed within its own bridge tube or chamber 20a that includes a frit bridge 20b. The counter electrode bridge tube 20a is filled with a solution 30c, which is preferably identical to the testing solution 30 used in the vessel 12 and the testing solution 30b used in the second bridge tube 24b of the reference electrode assembly 25, while the solution 30a used in the first bridge tube 24a of the reference electrode assembly 25 may be different depending, in part, upon the particular reference electrode 24.
Before a testing procedure in which the kinetics and mechanisms of electrode reaction will be investigated, the deposited material on the surface of the working electrode 16 needs to be cleaned. Therefore, the working electrode 16, which is coated with a chemical composition whose properties are to be tested, is inserted into the testing solution 30 in the glass vessel 12, and the reference electrode assembly 25 and counter electrode assembly are inserted into the glass vessel 12. The testing solution 30 is saturated with a suitable gas, such as argon or nitrogen, via the bubbling assembly 27 to purge the testing solution 30. Thereafter, the chemical composition on the working electrode is cleaned by the cyclic voltammetry in a desired potential region repeatedly. Thereafter, the solution is saturated with a required gas, such as oxygen or hydrogen, depending on the properties to be measured, through bubbling. Then, the RDE/RRDE is rotated at a stable, high speed by the motor 32. By sweeping a potential between working electrode 16 and reference electrode 24, a current is established between the counter electrode 20 and the working electrode 16 in the solution and is recorded.
While the aforementioned well-known testing apparatus and associated testing method has proven to be satisfactory and reliable, it suffers from several significant disadvantages. First, the testing procedure is relatively long (1–2 hours) and requires significant set-up in order to reliably reproduce the testing environment, which is vital to having reliable, repeatable results. Second, only one working electrode-mounted chemical composition may be tested during any given testing procedure. Third, the reference electrode may need to be calibrated between successive tests to account for drift of the reference potential, as may occur over time.
While these disadvantages are relatively minor when testing a small number of chemical compositions, they prove to be major disadvantages when testing thousands of compositions and wherein some of the thousands of compositions may need to be tested multiple times. Therefore, there exists a need in the art for an apparatus and method that permits multiple chemical compositions to be tested simultaneously in a three-electrode electrochemical cell.